As is known, in the last thirty years, and in particular after the publication by Charles Spindt of his first article on the manufacture of cold cathode vacuum tubes (C. A. Spindt et al., Physical properties of thin-film field emission cathodes with molybdenum cones, Journal of Applied Physics, vol. 47, December 1976, pages 5248-5263), there has been a renewed interest in the manufacture of high frequency, wide band, radiation insensitive vacuum tubes. This renewed interest is justified by the fact that this type of electronic devices, which, for generating an electron beam, exploit the field emission phenomenon instead of the thermionic phenomenon exploited by the conventional, old generation vacuum tubes, lend themselves to an ever increasing miniaturization.
In fact, the conventional vacuum tubes suffered from limitations due to the use of a thermionic cathode for electron emission, which cathode, in order to emit electrons, had to reach high operating temperatures of about 800 to 1200° C., with consequent problems linked to the management of the electrical power necessary to operate the vacuum tube (in a tube operating at low electrical power, namely less than 10 W, the electrical power necessary to heat up the cathode may be higher than the operating one) and of the so-called heating-up time (thermionic effect initiation time), and also linked to the stabilization of the control grid, which, in high frequency applications, was too close to the cathode (<25 μm) (see for example C. Bower, W. Zhu, D. Shalom, D. Lopez, G. P. Kochanski, P. L. Gammel, S. Jin, A micromachined vacuum triode using a carbon nanotube cold cathode, IEEE transactions on Electron Devices, Vol. 49, August 2002, pages 1478-1483).
On the contrary, the vacuum tube with a field emission array (FEA) cathode proposed by Spindt, generally known as Spindt Cathode, allowed the advantages provided by the vacuum electronics to be enjoyed, namely the property of the electrons of reaching higher speeds in the vacuum than in a semiconductor material. All these advantages are achieved with a substantially zero heating-up time, and with the possibility of arranging the control grid close to the cathode without having instability problems due to the heat of the electrodes, thus allowing higher operating frequencies to be reached (nominally from GHz to THz) and lower electrical power to initiate the electron generation process than necessary in thermionic tubes.
In particular, Spindt cathodes consist of microfabricated metal field emitter cones or tips formed on a conductive substrate. Each emitter has its own concentric aperture in an accelerating field generated by a gate electrode, also known as control grid, which is isolated from the substrate and the emitters by a silicon dioxide layer. With individual tips capable of producing several tens of microamperes, large arrays can theoretically produce large emission current densities.
Performance of Spindt cathodes are heavily limited by the destruction of the emitting tips due to their material wear, and for this reason many efforts have been spent worldwide in searching innovative materials for the production of the emitting tips.
In particular, the Spindt structure was improved by considering carbon nanotubes (CNTs) as cold cathode emitters (see for example S. Iijima, Helical microtubules of graphitic carbon, Nature, 1991, volume 354, pages 56-58, or W. Heer, A. Chatelain, D. Ugarte, A carbon nanotube field-emission electron source, Science, 1995, volume 270, number 5239, pages 1179-1180). Carbon nanotubes are perfectly graphitized, cylindrical tubes that can be produced with diameters ranging from about 2 to 100 nm, and lengths of several microns using different production processes. CNTs may be rated among the best emitters in nature (see for example J. M. Bonard, J.-P. Salvetat, T. Stöckli, L. Forrò, A. Châtelain, Field emission from carbon nanotubes: perspectives for applications and clues to the emission mechanism, Applied Physics A, 1999, volume 69, pages 245-254) and are ideal field emitters in a Spindt-type device, so many efforts have been spent worldwide in studying their field emission properties.
FIG. 1 shows a schematic view of a known Spindt-type cold cathode triode 1 including a cathode structure 2; an anode electrode 3 spaced from the cathode structure 2 by means of lateral spacers 4; and a control grid 5 integrated in the cathode structure 2. The cathode structure 2 with the integrated control grid 5 and the anode electrode 3 are formed separately and then bonded together with the interposition of the lateral spacers 4. The anode electrode 3 is made up of a first conductive substrate functioning as an anode, while the cathode structure 2 is a multilayer structure including a second conductive substrate 7; an insulating layer 8 arranged between the second conductive substrate 7 and the grid 5; a recess 9 formed to penetrate the grid 5 and the insulating layer 8 so as to expose a surface of the second conductive substrate 7; and Spindt-type emitting tips 10 formed in the recess 9 in ohmic contact with the second conductive substrate 7 and functioning as a cathode.